The present invention relates to a focusing control apparatus for an optical disk system, and more particularly, to a focusing control apparatus for changing the gain in a circuit according to a change in system gain, so as to maintain system stability.
To read the information from a compact disk or laser disk which is used for record/reproduction in an optical disk system, a tracking servo controls the tracing of the track formed on the surface of the disk. Also, to accurately focus a light beam on the track, a focus servo should be accomplished. Otherwise, it is difficult to read the recorded information correctly.
To read the recorded information, an optical pickup should be moved up and down in order to follow the minute vertical movement of the disk during rotation. If this vertical movement is accomplished, the data can be accurately reproduced. For this end, a focusing control is adopted.
In the optical disk apparatus, the surface of the recording medium is perpendicular to the beam's traveling direction. The optical-beam spot should always be focused on the surface of the recording medium during disk rotation. To meet this objective, it is necessary for the beam focusing to be accurately controlled in the vertical direction with respect to the disk's surface.
However, when a spindle motor is rotated by a motor control apparatus with the insertion of the disk in the driver of the optical disk system, the disk tends to vibrate vertically (parallel to the axial shaft) due to the skewing of the spindle motor or a warpage of the disk itself. At this time, the focusing control apparatus drives the movable objective lens along the axial direction, so that a laser beam passing through the objective lens keeps up with the planar vibration of the disk, to thereby maintain beam spot focusing on the surface of the disk.
FIG. 1 shows a schematic block diagram of the focusing control apparatus which performs the above-described control in an optical disk system. Referring to FIGS. 1 and 2, reference numeral 10 indicates an optical disk for reproducing the recorded data, 20 is a spindle motor for rotating the disk under the control of a servo (not shown), 30 is an optical pickup for detecting the recorded data by irradiating a light beam onto the surface of the disk, 40 is a focusing error detector, 50 is a phase compensator, 60 is an actuator driver, 80 is an actuator, and 65 is a subtracter for subtracting an output singal of actuator 80 from an disk plane vibration signal.
In an optical disk system constructed as above, disk 10 is rotated at a constant rotation speed by spindle motor 20, and optical pickup 30 irradiates the light beam onto disk 10 and supplies focusing error detector 40 with the focusing information obtained from the reflected light beam. Thus, error information can be detected.
Using the above-detected error information, phase compensator 50 compensates the phase of the input signal and actuator driver 60 drives the actuator installed on optical pickup 30 according to the compensation value, so that the focusing control for the vertical movement of the objective lens can be performed.
The focusing control executed by the focusing control apparatus of FIG. 1 will be described in more detail with reference to the construction of the compensator circuit shown in FIG. 2.
Referring to FIG. 2, when the vibration of the scanned disk surface scanned by the objective lens occurs and, accordingly, focusing is less than optimum, the amount of error is detected by focusing error detector 40 where it is converted into an electrical signal.
The structure of focusing error detector 40 is generally known. A conventional light-beam detector which is included in focusing error detector 40 and comprised of a quadrant photodiode or one divided into two sections, generates a current signal corresponding to the intensity of the light beam reflected from the disk. Next, the current generated by each detector section is converted into a voltage by a current-to-voltage converter, with the resulting voltage being input to a subtracting circuit. The output of the subtracter is the error signal. The error signal can be detected by a variety of other methods, for example, a knife edge method, an astigmatism method, the Puco method or a critical angle method.
The error signal is input to phase compensator 50 which produces the correction signal minimizing the input error. The correction signal is then input to actuator driver 60 which drives an actuator according to the input driving signal, so that the focus of the beam spot can keep up with the plane vibration of the disk at all times.
Here, phase compensator 50 should be designed such that it increases the system gain large enough in transmission characteristic of the overall loop, so as to not only restrict the error amount within the focusing depth, but also secure the sufficiently large phase at the gain cross frequency.
The gain and phase characteristics of phase compensator 50, which is a conventional circuit element, are shown in FIG. 3. As shown in FIG. 3, if the laser beam intensity varies during system operation, or the amount of reflected beam input to the photodiode changes due to fluctuations in the efficiency of the optical system or the reflective ratio shape of the disk being recorded or reproduced changes, the gain of the transmission characteristic of the overall loop changes, as illustrated by the graph of FIG. 3. In FIG. 3, curve 1 shows the case which the gain is increased, curve 2 shows the case which the gain is initially adjusted, and curve 3 shows the case which the gain is decreased.
Also, an error value .epsilon. between the plane on which the beam is focused and the disk surface is expressed as follows: ##EQU1## where R(t) is a plane vibration and Gt is an overall gain.
Therefore, if the overall gain Gt decreases as shown by curve 3 or if the plane vibration Rt exceeds a predetermined window, the error also will increase. Therefore, the error may be greater than the focusing depth. On the contrary, if the overall gain increases as shown by curve 1, the phase decreases so that the system stays in an unstable condition, which leads to oscillation.
For example, focusing error signals for system gain increase and system gain decrease are shown in FIGS. 4 and 5, respectively. When the system gain decreases, a the width of low frequency error signal becomes large, as shown in FIG. 4. In contrast, when the system gain increases, the error signal begins to fluctuate as shown in FIG. 5, with a higher oscillating frequency.
As described above, the conventional phase compensator circuit is furnished with a fixed voltage gain, so that, if laser beam intensity changes during operation of the system or the system uses disks having different reflectances, the overall gain characteristic of the system changes. Therefore, a decrease in gain results in error levels exceeding the focusing depth, to produce an out-of-focus condition. On the other hand, an increase in gain reduces the phase, so that the system oscillates.